Use of laser beam scanning apparatus in high-quality applications such as those employing modern color graphic techniques relates to accurate positioning of a laser generated dot relative to adjacent dots, as noted in the literature (e.g. Bestenheimer et al, Journal of Appl. Photo. Eng. vol 2, 1976). A system for improving the performance in these applications has been made the subject of our previous Israel Patent Application, Ser. No. 80242 filed Oct. 7, 1986.
One of the main problems in achieving the accurate positioning required by these applications is that the optical path of the laser beam may constitute layers of air which exhibit different indices of refraction due to thermal or pressure variations between them. This layering phenomenon varies slowly over time with the result that the laser beam passing through these layers will undergo varying deflections and will appear to originate from a plurality of slightly different light sources. Attempting to focus such a beam using a focusing lens will result in a fuzzy focal spot that will change with time according to the layer fluctuation.
The basis for the variations in the air layers through which the laser beam passes has been attributed to the contruction of the optical path. Since the optical path is generally constructed above a support surface integrally formed with the equipment, a thermal gradient is created between the air and the surface due to heat dissipation through the latter. Slow air currents will cause a slight alteration in the makeup of the air layers. Such phenomena can be seen near the surface of an asphalt road on a hot day. Even if the thermal variations are small between air layers, these layers generate different indices of refraction and the accumulation of the effect over long path lengths of the laser beam is perceptible.
The undesired effects of thermal variations in air layers is especially troublesome in the region of the optical path where the laser beam is at its maximum width. The wavefront of the laser beam is no longer of uniform phase but is instead corrugated so that a corrugated beam is produced, and these distortions change with time. Beam tracking in the range of 1 micron is made impossible under these conditions.
It would therefore be desirable to eliminate sources of distortion due to variations in the air layers located in the optical path of laser beam.